I played recently over an hour with Macos-X.
Mail.app is even not able to use plain SMTP on the PUBBOX.
Looks like CRAM_MD5, NTLM are not supported (at least not on this Mac). Mail.app simply does not state what it does, and why it does not work. I could see communications on port 25 with tcpdump, but no authentication occured.
Is it possible that Mail.app wants PLAIN authentication?
Are there other clients available besides Mail.app?